Height safety equipment is intended to prevent fall injuries to personnel working at height. A common arrangement for height safety equipment is for a safety rope or cable to be attached between end anchor points in an area to which access is required. Intermediate anchor points can then be added along the length of the cable to reduce fall distances before arrest and also to enable a change in direction along the length of a cable between end anchor points such that the cable can be routed around corners or some other direction. Personnel wear harnesses connected to travellers that are attached to and able to move freely along the safety ropes or cables. More sophisticated travellers are able to transverse intermediate anchor points without becoming detached from the cable.
The loads likely to be applied to anchor points depend on various factors. Probably the most significant of these is whether the height safety equipment is a restraint system intended simply to restrain personnel against the possibility of falling or whether the height safety equipment is a fall arrest system intended to arrest personnel safety immediately following a fall. Fall arrest systems allow personnel to access areas close to edges of building or tall structures whereas restraint systems limit movement of personnel to safe access areas where there is no possibility of a vertical fall. Therefore, maximum likely loading on anchor points in restraint systems is substantially less than that in fall arrest systems. This invention is intended for use in fall arrest systems although it could also be used in restraint systems.
One example of a commonly accessed fragile structure is roofs on relatively tall buildings. Fall arrest systems are installed to enable access to areas where falls are possible such as gutters and areas close to roof lights. Many such roofs exist on commercial buildings where the roof structure comprises pieces of relatively thin steel sheet formed with regular ridges that run typically from the apex of the roof along its pitch to the edge. Each piece is usually attached to adjacent pieces and all pieces are fixed to a system of lightweight formed steel beams that provide rigidity over the area of the roof and attach the roof to the surrounding ground bearing structure such as walls, pillars or external suspending structures. Whilst such roofs are intended to withstand the worst anticipated weather conditions they are not designed to cope with the relatively high localised loading that may be transmitted through fall arrest system anchor points.
The attachment of anchor point brackets to roofs by fall arrest system installers is most conveniently achieved by screwing the brackets directly to the roof sheets. This avoids the need to access the internal roof structure from outside the roof and also utilises the attachment techniques most commonly applied in the roof installation industry. Further, this allows the anchor point locations to be determined solely by the requirements of the fall arrest system because the anchor points can be located anywhere on the roof surface and are not limited to locations where they can be attached to the beams.
The structural strength of roofs formed of roof sheets is not very great. Therefore, where fall arrest system anchor points are attached to the roof sheets directly rather than to the support structure, it is important that the loading applied to the roof structure is limited.
Further, whilst screws used in roofing tend to provide good grip in shear, they pull out relatively easily in tension largely as a result of tearing in the thin steel roof sheet. Therefore, it is important where fall arrest system anchor points are attached to a roof, particularly with screws, that loading on the anchor point attachment to a roof is limited and primarily applies shear forces to attachment screws irrespective of fall conditions such as fall distances and number of personnel falling.
In order to arrest personnel safety following a fall, a fall arrest system needs to absorb all fall energy safely and without subjecting personnel to arresting loads greater than maximum safe values, which are specified by industry and international regulations. Also, most international regulations require that any load on any part of the system following the most demanding fall conditions for which a system is designed should never be greater than 50% of the load at which such a part fails. This safety factor is also applied to anchor point attachments and their supporting structures such as a wall or roof.
Cable such as steel wire commonly used in cable based fall arrest systems has very little elastic stretch and therefore absorbs little fall energy in the event of arresting a fall unless end anchor point loading is able to be relatively high. Personnel wear energy absorbing lanyards to limit arresting loads on personnel and these will assist to some extent in absorbing fall energy, although the deployment force of the lanyard absorbers is relatively low and also the deployment extent has the effect of adding to the fall distance and therefore the fall energy. However, anchor point loading on relatively fragile structures needs to be limited to a maximum of 50% of the strength capability of the structure itself. End anchor point loading in the event of fall energy absorption by elastic stretch alone would easily exceed 50% of the capability to failure to anchor point attachments to many roofs. Anchor point loading at a change in direction of the cable becomes significantly greater even than end anchor point loading. For example, an anchor supporting a ninety degree change in the direction of the cable would need to support a load increased by a factor to the square root of two.
A further condition that would cause high loading at an anchor point is where a multiple personnel fall occurs close to or on an intermediate anchor point. Initially, the multiple fall energy would need to be absorbed largely by reluctant extension at the anchor point itself to avoid high anchor loading. This depends on the degree of reluctance and extension. If the extension is low then loading at the anchor will be correspondingly high.
One problem encountered with energy absorbers, particularly for those used in fall arrest systems, is that the position at which a fall will occur relative to anchor point brackets cannot be predicted so that energy absorbers must be able to operate effectively for a fall arrest load being applied from a range of directions. Further, in order to allow economies of scale to be achieved by the use of common components throughout fall arrest systems and to avoid the possibility that a fall arrest system could be rendered ineffective by mounting an anchor bracket in the wrong orientation it is desirable to provide an energy absorber able to operate over a wide range of directions of applied load.
Accordingly, a first object of the invention is to provide an energy absorber capable of limiting loading to a known and safe value at its attachment to a structure irrespective of the direction of the loading.
A further object of the invention is to provide an energy absorber able to absorb a maximum or optimum amount of energy for a given extension and loading limit.